For determining flow characteristics, especially the flow, of a medium, respectively a fluid, thermal measuring devices are known, which make use of the fact that a (flowing) medium transports heat. The flow sensors have, in such case, a classic construction composed of at least one heating element and at least one temperature sensor, in order to determine the flow of the medium.
Known from the state of the art are so called anemometers, in the case of which the heating power is controlled in such a manner that a predetermined temperature is established on the at least one temperature sensor due to the heat transport of the medium. Based on the supplied, controlled, heating power, the flow of the medium can be determined.
Known, furthermore, from the state of the art are so called calorimetric flow sensors, which ascertain the flow of the medium by means of two temperature sensors and a heating element lying between the two temperature sensors. In such case, the medium is heated at the heating element and the temperature difference between the two temperature sensors determined, in order so to obtain the flow of the medium. In the case of this method, the heating power supplied to the heating element is not controlled. The present invention relates to such calorimetric flow sensors.
Due to layout- and manufacturing tolerances, as well as inhomogeneities in substrate and layer structure, temperature sensors formed on a substrate are not one hundred percent thermally and/or electrically stable. Such is necessary for an exact zero point. These deviations of the zero point, thus the zero point offset, can rise to one hundred percent of the actual measured value. Especially, in the case of microfluidic applications, which have a very small measuring range of 1 ml/min and less, a more exact and temperature stable zero point is of great importance, since this is permitted to deviate by only fractions of the measuring range.
For adjusting this zero point correspondingly, essentially two adjustment methods are known from the state of the art. In the case of the first adjustment method, the resistance bridge, which typically serves for evaluation of the temperature sensors, is adjusted by means of laser trimming of the temperature sensors. Disadvantageous in such case is that the laser trimming cannot be applied to all substrates, respectively to all layers, of the flow sensor. The second method is an electronic adjustment wherein an offset correction is performed in the signal path of the sensor electronics, such that in the case of a flow of zero the measurement signal is likewise essentially zero. Disadvantageous in this method is that a desired temperature stability, respectively temperature independence, of the zero point is only conditionally achieved.